1. Field of the Invention
The present disclosure relates generally to golf balls. Specifically, this disclosure relates to golf balls that include an inner core formed from a highly neutralized acid polymer, and an outer core that is also formed from a highly neutralized acid polymer, where the inner core does not include substantially any fillers but the outer core does include fillers.
2. Description of Related Art
Modern golf balls are known to be made from a variety of polymer materials. The material making up a golf ball may affect the golf ball's performance characteristics in several ways. For example, the selection of the material for use as a golf ball may affect the golf ball's coefficient of restitution, initial velocity off the tee, feel, durability over time, and other properties.
Suitable known materials for use in a golf ball include thermoset materials, such as rubber, styrene butadiene, polybutadiene, isoprene, polyisoprene, and trans-isoprene. Known materials also include thermoplastics, such as ionomer resins, polyamides or polyesters, and thermoplastic polyurethane elastomers. Suitable materials also include polyurea compositions, as well as other materials.
In particular, ionomers are often used for to form the various structural components of known golf balls. For example, ionomers such as Surlyn™ available from E.I. DuPont de Nemours & Company are known to be used for cover layers of golf balls. Other types of ionomers, generally referred to as highly neutralized acid polymers, may also be used in golf balls.
Specifically, highly neutralized acid polymers are known to be used as the material for a golf ball core. For example, U.S. Pat. No. 6,756,436 to Rajagopalan et al., entitled “Golf Balls Comprising Highly-Neutralized Acid Polymers” and filed Apr. 9, 2002, discloses golf balls having highly neutralized acid polymer cores. The disclosure of this patent is hereby incorporated by reference. Other conventional highly neutralized acid polymers are generally disclosed in U.S. Pat. No. 7,652,086 to Sullivan et al., entitled “Highly-neutralized Thermoplastic Copolymer Center for Improved Multi-layer Core Golf Ball” and filed Feb. 3, 2006, the disclosure of which is hereby incorporated by reference.
In some known golf ball constructions, a multi-piece golf ball may include both an inner core and an outer core. For example, U.S. Pat. No. 7,468,006 to Sullivan et al. discloses a golf ball having an inner core and an outer core. In particular, this patent teaches that one layer is made from a relatively soft highly neutralized acid polymer composition, and another layer is made from a relatively hard highly neutralized acid polymer composition. The disclosure of U.S. Pat. No. 7,468,006 to Sullivan et al. is hereby incorporated by reference.
The compositions of an inner core and outer core, or other layers of the golf ball, may be manipulated to achieve desired effects. For example, U.S. Pat. No. 7,651,415 to Ladd et al. discloses a golf ball with a core, intermediate layer(s), and cover layer(s) having a density gradient in its inner layers. The density of the innermost layer can be lowered with a density reducing filler, while outer layers include a density increasing filler. The disclosure of U.S. Pat. No. 7,651,415 to Ladd et al. is hereby incorporated by reference.
The selection of material(s) in the core, as well as the golf ball's overall construction, may affect the coefficient of restitution (“COR”) in particular. As is widely known, the COR of a golf ball is a measure of how efficiently the golf ball transfers kinetic energy. A high COR value means that, when the golf ball is struck by a golf club, the golf ball transfers more of the kinetic energy of a golf club into forward movement by the golf ball. A high COR value may achieve advantageous play characteristics such as an increased initial velocity, or an increased total distance.
Therefore, there exists a need in the art for a golf ball having an increased COR that may also incorporate the advantages of using highly neutralized acid polymers to form an inner and outer core.